The present invention relates to negative-electrode active material for rechargeable lithium battery, method of preparing the same, and rechargeable lithium battery comprising the same.
A battery generates electrical power using materials capable of electrochemical reaction for positive electrode and negative electrode. A representative example of the battery is rechargeable lithium battery which produces electrical energy by change in chemical potential at intercalation/deintercalation of lithium ion in positive electrode and negative electrode.
The rechargeable lithium battery is prepared using materials capable of reversible intercalation/deintercalation of lithium ion as positive- and negative-electrode active materials, by filling organic electrolyte or polymer electrolyte between the positive electrode and negative electrode.
As positive-electrode active material for rechargeable lithium battery, lithium complex metal compound is used, and for examples, complex metal oxides such as LiCoO2, LiMn2O4, LiNiO2, LiNi1-xCoxO2 (0<x<1), LiMnO2, etc. are studied.
As negative-electrode active material for rechargeable lithium battery, carbon materials of various forms including artificial graphite, natural graphite and hard carbon capable of intercalation/deintercalation of lithium have been used. Especially, graphite has low discharge voltage of −0.2V vs lithium, and thus, battery using it as negative-electrode active material shows high discharge voltage of 3.6V. Thus, the graphite active material provides advantage in terms of energy density of lithium battery, and secures long life of rechargeable lithium battery due to excellent reversibility, and thus is most widely used. However, graphite active material has problems in that when preparing an electrode plate, capacity is low in terms of energy density per unit volume of the electrode plate due to low density of graphite (theoretical density 2.2 g/cc), and there is riskiness of ignition or explosion due to mis-operation and overcharge of battery because side reaction with organic electrolyte easily occurs at high discharge voltage.
Thus, inorganic active materials such as Si have been studied. The Si inorganic negative-electrode active material is known to form Li4.4Si to show high lithium capacity of about 4200 mAh/g. However, it causes large volume change of 300% or more at intercalation/deintercalation of lithium, namely at charge/discharge. Thereby, pulverization of negative-electrode active material occurs, and the pulverized particles are condensed to cause electrical deintercalation of negative-electrode active material from current collector. The electrical deintercalation may largely decrease capacity retention ratio and cycle life property of battery. Therefore, in order to inhibit volume change of inorganic negative-electrode active material, studies for preparing carbon/Si nanoparticle composite to use it as negative-electrode active material have been progressed. In the carbon/Si nanoparticle composite, carbon functions as electrical conductor, thus improving capacity retention ratio of battery to some extent. However, in order to obtain relatively excellent capacity retention ratio, carbon content should exceed 50 wt % in the composite, which may lower capacity itself, and even if excessive amount of carbon is included, capacity decreases to less than 1500 mAh/g after 50 cycles.